This invention relates to metal surface finishing and particularly to an improved apparatus and method for microfinishing metal surfaces using coated abrasive tape materials.
Numerous types of machinery components must have finely controlled surface finishes in order to perform satisfactorily. For example, surface finish control, also referred to as microfinishing, is particularly significant in relation to the manufacturing of journal bearing and cam surfaces such as are found in internal combustion engine crankshafts, camshafts and power transmission shafts and other finished surface. For journal type bearings, very accurately formed surfaces are needed to provide the desired bearing effect which results when lubricant is forced between the journal and the associated bearing. Improperly finished bearing surfaces may lead to premature bearing failure and can limit the load carrying capacity of the bearing.
Currently, there is a demand for higher control of journal bearing surfaces by internal oombustion engine manufacturers as the result of; greater durability requirements necessary to offer improved product warranties, the higher operating speeds at which engines (particularly in automobiles) are now required to sustain, and the greater bearing loads imposed through increased efficiency of engine structures.
In addition to bearing structures, surface finish control must be provided for engine cylinder walls in order to provide the desired oil and gas seal with the piston rings. Numerous other types of machine components also require controlled surface finishes, particularly along areas of sliding contact between parts.
Microfinishing has primarily been accomplished according to the prior art using several different types of machining techniques. In stone microfinishing, a stationary honing stone is brought against the desired surface. When microfinishing cylindrical journal bearing surfaces, the honing stone is caused to oscillate traversely from one edge of the journal to another as the workpiece is rotated with respect to the stone. This possesses a number of significant disadvantages. Due to the requirement that the honing stone be soft enough to be self-dressing and to provide the desired material removal characteristics, the stone, through use, takes on the shape of the part being finished. Therefore, this method, instead of correcting geometry variations in the part being microfinished, actually causes such variations to occur. Additionally, since honing stones are perishable, they must be frequently replaced and redressed. Finally, it is extremely difficult to find honing stones with consistent qualities resulting in significant differences in the finished parts when machined by different stones.
Another significant disadvantage of stone microfinishing of journal bearings using a honing stone is the fact that, since the journals generally include outwardly projecting radius edges, the stone cannot laterally overstroke portions of the surface being machined which leads to uneven stone wearing. Such uneven wearing causes a change in the profile shape of the honing stone, and this shape is consequently generated in subsequent parts being machined. Finally, since the honing stone generally has sharp corner edges, it cannot be used to microfinish near the radius edges of the bearing surface.
In another known microfinishing process, herein referred to as conventional coated abrasive tape microfinishing, the surface being finished is caused to rotate and a coated abrasive tape is brought into contact under pressure with this surface. As the part is rotated, the abrasive material reduces the roughness of the surface. In the conventional process, the tape is brought into contact with the rotating surface by pressure exerted by compressible elastomeric inserts, typically made from urethane plastic compounds. The conventional coated abrasive tape microfinishing process overcomes several of the disadvantages associated with stone microfinishing. This process is capable of microfinishing in the journal fillet radius area since the tape is relatively flexible. In addition, this process uses a renewable abrasive surface which can be purchased having consistent qualities. This process, however, does not overcome other disadvantages of stone microfinishing. Principal among these disadvantages of this process is the fact that the process does not correct geometry variations in the part being microfinished, since the insert backing the coated abrasive tape is a flexible material and therefore, the tape conforms to the surface profile of the component surface being machined.
In still another variation of microfinishing processes known to the prior art, a rigid insert is used to press abrasive coated paper or cloth material into contact with a relatively moving workpiece surface. Abrasive coated paper or cloth materials are, however, relatively thick and oompressible, and therefore, this method did not enable significant workpiece geometry corrections since the paper or cloth would "give" and conform to minute irregularities in the workpiece surface.
In addition to the above-noted shortcomings according to the currently known microfinishing processes, great difficulty has been encountered in removing ferrite caps which are present on the finished surfaces of nodular iron workpieces. These hard caps are present on the outside surface of the bearing and can lead to premature bearing failure.
In view of the above-described shortcomings of microfinishing devices and methods acoording to the prior art, it is a principal object of this invention to provide a microfinishing apparatus and method which is capable of correcting geometry imperfections in finished surfaces. It is yet another object to consistently prcduce surfaces having smoothness characteristics superior to those achievable by conventional means.
The above principal objects of this invention are provided by a microfinishing system which employs an abrasive coated tape which is brought into contact with a rotating workpiece, and is pressed into contact by that wcrkpiece by a rigid precision formed backup insert. This rigid insert does not cause the abrasive tape to conform to the surface profile of the workpiece. Instead, the rigid insert causes greater abrasive tape contact pressure to be applied to portions of the wcrkpiece surface which extend beyond the desired surface, thereby causing greater material removal in those areas. This system therefore permits the microfinishing system to correct geometry imperfections in the workpiece. In the practice of this invention, it is essential that the abrasive coated tape be made of a material which is relatively incompressible such that the tape will not conform to irregularities but instead will enable these irregularities to be removed. Since the insert is not the primary cutting tool, it is not subject to significant changes in profile with use. With appropriate additional components, the rigid inserts may be provided with the capability of polishing fillet radius areas. The microfinishing system according to this invention has been found to provide a significant advance in the art of microfinishing enabling consistent production of surface finishes unachievable using the devices and processes according to the teachings of the prior art.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates upon a reading of the described preferred embodiments of this invention taken in conjunction with the accompanying drawings.